AC to DC rectifiers are widely used to convert AC line electric power to DC power to be used by inverters (for motor, UPS, and other applications), DC/DC converters, and passive loads such as resistors. In any rectifier circuit, the AC line voltages are rectified and ripple of the rectified voltage is filtered using a parallel capacitor and occasionally a series inductor. This results in a fixed, i.e., ripple free, DC voltage.
Without appropriate pre-charge circuitry, the start-up transients can be harmful to the systems. If the AC line and/or the DC link filters do not have sufficient impedance, significant switching transient may occur upon closing a three-phase supply switch. A large current surge charges the capacitor and depending on the system impedance, the surge current can reach prohibitive levels. As a result, the rectifier switches and the filter components (inductors and DC link capacitor) may fail due to the excessive current/voltage through them. The transients may also create electromagnetic interference that may interfere with other equipment in the power system and can lead to a catastrophic failure. Therefore, during start-up it is mandatory to establish a high impedance path between the large AC line voltages and the DC link capacitor. This task can be accomplished by a pre-charge or soft charge circuit that is placed in series with the DC link output of the rectifier. The main task of the soft charge circuit is to exhibit sufficiently high impedance during start-up and zero impedance during normal operation.
Known voltage source inverters (VSI) that have a large DC link capacitor filter use a resistor-contactor arrangement to limit the inrush current into the capacitors, and thereby provide a means to soft-charge the DC link capacitor CDC, see FIG. 1(a). Because of the mechanical nature of the contactor, the reliability of the variable frequency drive (VFD) is adversely affected. Moreover, the time delay involved in the basic response of the contactor can result in an unfavorable sequence of events during a brown out condition. Given these facts, the soft-charge circuit is often considered to be the weakest part of an otherwise well designed VFD.
The typical prior art VFD system shown in FIG. 1(a) employs the soft charge circuit including a resistor RSC and a contactor switch MC connected in parallel. The resistor RSC is sized for the start-up charging transient while the contactor switch MC is sized for the normal operation. During start-up the contactor switch MC is open (not conducting) and it remains so until the DC link capacitor voltage reaches a critical value (roughly near rated operating voltage). Once the critical voltage level is reached, then the contactor switch MC is closed, and the resistor RSC is by-passed.
The rectifier system of FIG. 1(a) exhibits high energy-efficiency because the contactor switch MC has very low conduction losses and the large pre-charge transients are limited to less harmful levels. If for any reason the input AC supply experiences a large dip either due to brown out condition or due to a large load being suddenly applied across the AC supply, there is a possibility that the soft-charge contactor does not open and remains closed. When the input AC supply recovers, the resulting surge current can be large and damage the input rectifiers, and the DC link capacitor. When large current flows through the soft charge resistor during such events, the contacts can even melt and fuse together, rendering them useless for future use. Hence, by nature, this approach does not yield a highly reliable solution. Also, due to mechanical actuation, the mechanical contactor switch wear-out is rapid and inevitable. Therefore, the life of the contactor is limited and in general much shorter than most of the stationary electrical parts inside a rectifier system.
There have been suggestions of replacing the magnetic contactor MC in FIG. 1(a) with a semiconductor switch, as shown in FIG. 1(b). However, the semiconductor switch requires intelligent control logic circuitry and is associated with steady-state power loss.
Thyristor controlled rectifiers have been used in VFDs but the additional gate circuit adds cost and increases the component count, which reduces reliability. With one known topology, the input rectifiers are replaced by thyristors. The triggering angle of the thyristors is controlled in such a manner that the DC link capacitor charges up smoothly with no inrush. When a brown out occurs, the thyristor angle is such that it provides the maximum output voltage possible, similar to a typical diode bridge. When the voltage recovers after a brown out condition, the difference between the peak value of the input voltage and the DC link voltage is large enough to force the triggering angle to increase and thereby reduce the high inrush current. The technique, shown in FIG. 2, is well established and is used by some VFD manufacturers. However, this VFD needs six pack thyristor modules, which can be expensive, especially for small sizes due to low volume of production by semiconductor manufacturers. The VFD needs six gate-trigger circuits along with sensing and decision making logic (The trigger circuits along with the necessary logic occupy space and are expensive). The thyristors may cause a voltage notching effect if the regulated output voltage is lower than that achievable from the input ac source—this will require the use of input ac inductor that occupies space and is an added cost. Finally, gate drive and logic circuits reduce mean time between failures (MTBF) due to the increased component count in the VFD.
A second alternative topology uses a Magneto Resistive (MR) device that shows high resistance under the influence of large magnetic field and low resistance when the magnetic field resets to a lower level. The MR element could be connected in series with the DC link capacitor to soft charge it at start up or during the recovery time after a brown out condition. The circuit configuration is shown in FIG. 3(a) and the conceptual schematic in FIG. 3(b).
The present invention is directed to solving the problems discussed above, in a novel and simple manner.